1. Field of the Invention
The present invention relates to a method and an apparatus for detecting changed regions within dynamic images by using a prescribed noise model.
2. Description of the Background Art
In general, for processing dynamic images, a method using difference images is widely used. Here, the difference image is obtained by taking a difference in each picture element between images taken at sequential timings separated by a certain time interval dt.
In this method using difference images, it is possible to detect a movement of a target moving object by obtaining the successive difference images for images of the moving target, and utilizing the fact that the differences in the picture elements become large only in immediately the surrounding regions of the target moving object in a case the target moving object moved. One assumes that only the target moving object can move, while the background remains static and the imaging device is fixed. The actual detection of movement of the target moving object can be accomplished by applying various image processing operations such as binarization and labelling to the difference images.
Now, this conventional method for detecting a movement of a target moving object by using difference images is based on an assumption that the image of the background remains static throughout the entire operation. However, in reality, the image of the background is constantly fluctuating in accordance with a lighting condition, even for the completely motionless background. Namely, the background image is affected by the flickering of the fluorescent lamp usually employed at an indoor image taking site. Normally, the fluorescent lamp flickers at a frequency of the electric power source frequency such as 50 Hz or 60 Hz. The background images, thus can have lightness variations depending on the image taking timings.
For this reason, the picture element values in the difference image may not necessarily be small enough to be negligible for the background regions, and therefore it is necessary to separate the target moving object from the background regions by using an appropriate binarization threshold.
As a consequence, in order for movement of the target moving object to be detectable, it is necessary for the target moving object to have a certain level of contrast with respect to the background, so that it has practically been difficult to detect a movement of target moving object which is semi-transparent and has very little contrast, such as steam.
Conventionally, this problem has been resolved by employing an inverter circuit in a lighting equipment such that the flickering frequency of the fluorescent lamp is increased in order to reduce the influence of the flickering. However, it can be quite expensive to make such a modification to all the fluorescent lamps located within an entire view field to be imaged by the imaging device.
Moreover, controlling the image taking timings in synchronization with the appropriately sensed flickering frequency of the lighting equipment, it becomes impossible to take the images at a rate faster than the flickering frequency of the lighting equipment.